Crystalline polyamide resins, such as nylon 6 and nylon 66, have been widely used as fibers for clothes and engineering plastics for the production of automobile parts, machine parts, and parts of electric and electronic equipments, because they are excellent in toughness, chemical resistance, and electrical characteristics and easy to melt mold and process. However, the usable range of such resins in the above application fields is limited by their insufficient heat resistance, poor dimension stability due to water absorption, and insufficient mechanical strength. Particularly, in the application to automobile parts in which plastic parts are rapidly replacing metallic parts and the application to parts of electric and electronic equipments for use in surface mount technology (SMT) which is rapidly progressing with the advance of semiconductor technology, high performance which is difficult to achieve by known polyamide resins is required. Therefore, a polyamide resin excellent in heat resistance, dimension stability, and mechanical property is required.
As compared with known polyimide resins, the aromatic ring-containing polyamide resin (hereinafter may be referred to as “nylon MXD”) produced from m-xylylenediamine and adipic acid is characterized by high strength, high modulus of elasticity and low water absorption, and therefore, comes into use for the production of automobile part and parts of electric and electronic equipments which are required to be light in weight and small in size as substituents for metallic parts. In addition, the aromatic ring-containing polyamide resin has high gas barrier properties and finds its use for packaging materials, such as food packaging materials, which are required to prevent the permeation of gas.
As compared with nylon 6 and nylon 66, the crystallization speed of nylon MXD 6 is low. Therefore, nylon MXD 6 alone is difficult to crystallize in the mold for injection molding, this making the thin-wall molding difficult or likely to cause problem of deformation of molded article or decrease in mechanical strength. Therefore, to mold nylon MXD 6, it is required to increase the crystallization speed by blending a crystallization accelerator, such as nylon 66 and talc powder each having high crystallization speed, or improve the moldability by raising the mold temperature (Patent Document 1). However, as compared with the use of nylon MXD 6 alone, the properties change largely in a moist atmosphere when nylon 66 is blended, and the mechanical strength is reduced when talc powder is blended. Therefore, the blending amount is limited.
To remove this drawback, it has been proposed to enhance the crystallizability by introducing a rigid molecular structure into the molecular chain of polyamide. For example, a polyamide resin produced by the copolymerization of a diamine component composed of 15 to 65 mol % of p-xylylenediamine and 85 to 35 mol % of m-xylylenediamine and a dicarboxylic acid component composed of 45 to 80 mol % of a α,ω-aliphatic dicarboxylic acid having 6 to 12 carbon atoms and 20 to 55 mol % of an aromatic carboxylic acid such as terephthalic acid has been proposed (Patent Document 2). Although the crystallizability can be increased by increasing the content of p-xylylene, the melting point is excessively increased and the polyamide resin is likely to be thermally degraded during melt polymerization and melt processing. Therefore, the content of p-xylylene cannot be increased enough. If the content of the aromatic dicarboxylic acid is increased, the melt viscosity is excessively increased to reduce the molding and processing ability.
A wholly aliphatic polyamide produced from adipic acid and 1,4-butanediamine (hereinafter may be referred to as “nylon 46”) is heat-resistant and has a good moldability suitable for the production of thin-wall or small-sized parts because of its high melting point, high crystallinity, and relatively low melt viscosity. However, its high water absorption reduces various properties such as dimension stability under conditions for practical use.
A semiaromatic polyamide produced from terephthalic acid and 1,6-hexamethylenediamine (hereinafter may be referred to as “nylon 6T”) has a melting point of around 370° C. which exceeds its decomposition temperature. Therefore, the melt polymerization and melt molding are difficult, this making nylon 6T unsuitable for practical use. It has been proposed to reduce the melting point to a moldable temperature range, for example, around 320° C., by copolymerizing adipic acid, isophthalic acid or ε-caprolactam in an amount of about 30 to 40 mol % (Patent Documents 3 and 4). However, the crystallization speed and crystallinity to be attained are reduced by the copolymerization, this deteriorating several properties, for example, rigidity at high temperatures, chemical resistance and dimension stability as well as elongating the molding cycle to reduce productivity. In addition, the moldability is poor because the viscosity is likely to be reduced when kept in a molten state.
A semiaromatic polyamide produced from terephthalic acid, 1,9-nonamethylenediamine and 2-methyl-1,8-octamethylenediamine (hereinafter may be referred to as “nylon 9T”) has been proposed (Patent Document 5). As compared with nylon 6T, nylon 9T has a high crystallization speed, high crystallinity to be attained, and low water absorption and put into practical use. However, nylon 9T involves the same problems as mentioned above, i.e., properties are deteriorated by copolymerization, the melt flowability is lowered because the main component is aromatic dicarboxylic acid, an extremely high mold temperature is needed to make the molding difficult, and the productivity is low.
Patent Document 6 discloses a polyamide produced from a diamine component composed of an amount of p-xylylenediamine and a smaller amount of m-xylylenediamine and a dicarboxylic acid component composed of a straight-chain aliphatic dicarboxylic acid having 11 or more carbon atoms. This document teaches that a polyamide constituted by p-xylylenediamine and a straight-chain aliphatic dicarboxylic acid having 10 or less carbon atoms has a poor processability and is not suitable for practical use, because the melting point is close to the decomposition point.    Patent Document 1: JP 54-32458B    Patent Document 2: JP 3456501B    Patent Document 3: JP 64.11073B    Patent Document 4: JP 3-56576B    Patent Document 5: JP 3242781B    Patent Document 6: JP 47-15106B